The invention relates to a multi-layer eddy current probe for use in an eddy current test unit for non-destructive testing of electrically conductive materials, to a method for producing a multi-layer eddy current probe of this kind, and to an eddy current test unit comprising at least one multi-layer eddy current probe of this kind.
Eddy current testing has proven successful for non-destructive testing (NDT) of electrically conductive materials in many fields of application, for example for automated non-destructive testing on semi-finished products for the metal-producing and metal-processing industry, for carrying out tests on safety-related and functionally critical components for land vehicles and aircraft or in plant construction. The sensor systems employed or used for eddy current testing are usually called “eddy current probes”.
During eddy current testing, an eddy current probe is arranged at a short test distance from a surface of a test object, which surface is under test. The test object comprises an electrically conductive material at least in the region of the surface. A relative movement between the test object and the eddy current probe is generally generated for testing purposes, for example by the eddy current probe moving over that region of the surface which is under test.
An eddy current probe which is constructed using coils comprises one or more field coils (or excitation coils) which are connected to an AC voltage source in order to carry out the test and can then produce a radiofrequency electromagnetic alternating field which, during testing, penetrates the test material and produces eddy currents substantially in a layer of the test material which is close to the surface, said eddy currents having a retroactive effect on one or more measuring coils (or receiver coils) of the eddy current probe due to mutual induction. A defect in the tested region, for example a crack, an impurity or another material inhomogeneity, interferes with the propagation of the eddy currents in the test material and therefore changes the eddy current intensity and as a result the intensity of the magnetic secondary field, which has a retroactive effect on the measuring coil, too. The resulting changes in impedance in a measuring coil lead to electrical measuring signals which can be evaluated by means of an evaluation unit in order to identify and to characterize defects. Eddy current probes in which the field coils (excitation coils) and the measuring coils, which are separate from said coils, are coupled by means of the material of the test object are called transformer-type eddy current probes.
Whereas older-generation eddy current probes were typically constructed with coils wound from wire, eddy current probes which can be produced using printed circuit board technology methods in the manner of printed circuits are additionally also used nowadays. Said eddy current probes include multi-layer eddy current probes of the generic type which have a multi-layer arrangement with a large number of flat spiral coils which are arranged in different coil layers of the multi-layer arrangement, wherein a respective insulating layer which is composed of an electrically insulating material is arranged between immediately adjacent coil layers.
Each of the flat coils has an inner terminal on the inner turn and an outer terminal on the outer turn of the flat coil. The terminals serve to electrically connect the flat coils to other flat coils or external components. In order to electrically connect flat coils from different layers to one another, selected terminals of selected flat coils of different coil layers are electrically connected to one another by means of vias. A via (“vertical interconnect access”) is a vertical electrical connection, that is to say an electrical connection which runs substantially perpendicular to the layers, between flat coils in different coil layers. The connection can be realized, for example, by an internally metalized hole in the material of the insulating layer.
U.S. Pat. No. 5,389,876 (also see EP 0 512 796 B1) describes multi-layer eddy current probes which have a large number of identical flat coils which are encapsulated in a flexible, multi-layered integrated eddy current probe arrangement. One embodiment (FIGS. 6a and 6b) provides coil pairs with in each case two flat coils which are arranged one above the other in different coil layers. The turns of the two flat coils run with opposite winding senses in relation to one another. The inner terminals of the flat coils are connected to one another by means of a via. As a result, the flat coils of the coil pair are connected electrically in series, as a result of which a current flows through the turns of the two flat coils in the same circulation direction at a specified time, so that the fields of the two flat coils are added when an AC voltage is applied. In another embodiment (FIG. 8), two coil pairs of this kind are vertically interleaved one in the other such that a flat coil of a second coil pair is arranged between the flat coils of a first coil pair in the vertical direction. The associated flat coils of a coil pair are in each case connected by means of an inner via, which inner vias connect the inner terminals of the associated flat coils, so that in each case two vias pass through the turn-free inner regions of the two internal flat coils.
EP 2 056 103 B1 likewise describes flexible multi-layer eddy current probes with coil pairs which each comprise two flat coils which are arranged one above the other in different coil layers, wherein the turns of the two flat coils run with opposite winding senses in relation to one another and the inner terminals of the flat coils are connected to one another by means of a via.